The present invention relates to an electrode structure for nitride III-V compound semiconductor devices and, more particularly, to a Schottky electrode structure having high adhesion strength and good temperature characteristics.
A conventional hetero-junction field effect transistor (hereinafter referred to as xe2x80x9cHFETxe2x80x9d) made of nitride semiconductor is generally of such a construction as shown in FIG. 9. As is shown in FIG. 9, the HFET includes a sapphire substrate 101, a low temperature grown GaN (gallium nitride) buffer layer 102 having a layer thickness of 20 nm, and a GaN buffer layer 103 having a layer thickness of 2 xcexcm and a carrier density of 5xc3x971016 cmxe2x88x923, the latter two layers being sequentially placed on the substrate. Sequentially stacked on the buffer layer 103 are an AlGaN (aluminum gallium nitride) spacer layer 104 having a layer thickness of 20 nm, an AlGaN donor layer 105 having a layer thickness of 20 nm and a carrier density of 1xc3x971018 cmxe2x88x923, and a GaN contact layer 106 having a layer thickness of 10 nm and a carrier density of 2xc3x971018 cmxe2x88x923.
Source/drain electrodes 107, 107 using an ohmic contact, and a gate electrode 108 using the Schottky junction are formed on the GaN contact layer 106.
Generally, metals having a large work function, such as nickel (Ni) (Y.-F. Wu et al, IEEE Electron Device Lett. 18 [1997] 290), platinum Pt (W. Kruppa et al, Electronics Lett. 31 [1995] 1951), and gold Au (U.S. Pat. No. 5,192,987), have been used as Schottky electrode materials for gate electrodes. These metals are ohmic electrode materials relative to p-type semiconductors and are, therefore, used as Schottky electrode materials relative to n-type semiconductors.
However, these metals will show relatively weak adhesion to the semiconductor and, at temperatures of 400 xc2x0 C. or more, the metals will give rise to the problem of increased current leaks, with the result that the HFET is very much deteriorated in its characteristics.
Therefore, an object of the present invention is to provide an electrode structure for nitride III-V compound semiconductor devices, the electrode structure including a Schottky electrode having a high adhesion to a semiconductor and good temperature characteristics.
In order to solve the above object, present inventors made an extensive research and, as a result, it was found that the electrode structure described below would be effective as a solution. This finding led to the present invention.
That is, in nitride III-V compound semiconductor devices, it was found that a nitride o f a metal having a nitride forming negative free energy could provide a Schottky electrode showing a high adhesion to semiconductors and good temperature characteristics. The reason for this is that the formation of the metallic nitride on a nitride semiconductor leads to the formation of a chemical bond through nitrogen atoms, resulting in a stronger bond than prior art semiconductor/metal interfaces.
Therefore, an electrode structure for nitride III-V compound semiconductor devices in accordance with the present invention is characterized in that a metallic nitride is used as an electrode material, a metallic material of the metallic nitride having a negative nitride formation free energy.
The metallic nitride should show a metallic conductivity in order to play a role of an electrode.
As examples of metals having a negative nitride formation free energy and at the same time forming a metallic nitride showing a metallic conductivity, mention may be made of metals included in the IVa, Va, and VIa groups. Such metals are exemplified by titanium (Ti) and zirconium (Zr) belonging to the IVa group, vanadium (V), niobium (Nb) and tantalum (Ta) belonging to the Va group, and chromium (Cr), molybdenum (Mo), and tungsten (W) belonging to the VIa group. Hafnium (Hf) is an exception and use of this material is undesirable because its nitride formation free energy is positive. As Table 1 given below tells, the tabulated data of metals shown indicates that all of the metals show a negative nitride formation free energy. The larger the formation free energy in the negative direction, the better. The reason for this is that the resulting metallic nitride is more stable and, in particular, Zr, Ti, Ta, and Nb having a formation free energy of not more than xe2x88x9250 kcal/mol are preferred.
The metal material for these metallic nitrides may be a single metal or a composite metal comprised of two or more kinds of metals. These metals have a high melting point and, accordingly, nitrides of the metals have a high melting point and are thermally stable, being thus able to exhibit good temperature characteristics.
From the standpoint of thermal stability, it is desirable that the melting points of the metals and metallic nitrides be as high as possible, while some correlation can be observed between the melting points of metals and the melting points of metallic nitrides through the formation free energy. That is, in case that even if the melting point of a metal is relatively low, but if its formation free energy is large, the melting point of a nitride of the metal tends to rise. Therefore, from the standpoint of thermal stability, Zr, Ti, Ta, Nb are preferred.
For depositing such a metallic nitride, various methods, such as molecular beam epitaxy using a nitrogen radical and a reactive sputtering method, can be employed.
A suitable thickness range of the metallic nitride layer formed in this way is not less than 10 nm but not more than 200 nm. If the thickness of the metallic nitride is less than 10 nm, the metallic nitride layer does not form a continuous layer, and this poses a problem that no satisfactory reproducibility could be obtained with respect to the characteristics of the metallic nitride layer. On the other hand, the thickness of the metallic nitride which is more than 200 nm will cause a problem of deterioration of the electrical characteristics and crystallinity of a GaN semiconductor layer due to the stress of the metallic nitride layer.
Further, in order to facilitate bonding of lead wire onto the metallic nitride layer, a layer comprised of Au or an Au alloy may be placed on the metallic nitride layer. The Au alloy is not particularly limited; as long as it is superior to Au in hardness, the alloy is acceptable. As a method of depositing Au or an Au alloy, vacuum deposition and sputtering may be mentioned, but are not limitative. By depositing Au or an Au alloy on the metallic nitride, it is possible to reduce the contact resistance of a contact portion between the electrode and the lead wire and hence generation of heat from the contact portion to thereby further improve the characteristics of the electrode.
Thus, a schottky electrode having high film adhesivity and a good temperature characteristic can be obtained.